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272Crystals formed through biological activity

reporting that synthetically grown calcite crystals exclusively show rhombohedral

Habitus, but, when an organic compound is added as an impurity, a scalenohedral

Habitus is exclusively formed. The reason why calcite crystals forming sea urchin spicules have a scalenohedral Habitus is probably due to the cooperation with protein in their growth. This is in agreement with the spicule shape.

There is no doubt that the Habitus of calcite crystals forming the exo-skeleton of coccoliths is designed naturally to match the function of spicules, but further investigation may be required to explain the growth mechanism of this peculiar form and the relations between R and V crystals.

14.2.3 Magnetite

Magnetite (Fe3O4) is a spinel group mineral belonging to the cubic system m3m, and it shows a wide mode of occurrence as a rock-forming mineral, but it also occurs in living bodies. Earliest attention centered on magnetite crystals found in the cells of magnetotactic bacteria. Later, magnetite crystals were found in the brain cells of pigeons, dolphins, and salmon, suggesting that the crystals are possibly responsible for the homing instincts of pigeons and the behavior shown by fish that return to their spawning grounds. The adult teeth of chiton (marine mollusks) also consist of magnetite (the immature teeth are composed of goethite,

-FeOOH, and lepidocrosite, -FeOOH, which transform into magnetite as the chiton matures).

The sizes of magnetite crystals formed in the cells of magnetotactic bacteria, fish, and pigeons are 700 Å across, corresponding to one magnetic domain size, and they form a rosary chain. The alignment of individual crystals forming the chain is characteristic of biological magnetite. Individual magnetite crystals are idiomorphic and polyhedral crystals; skeletal or dendritic morphology, which represents crystal formation under a high driving force, has not been observed. However, the morphology of polyhedral crystals is markedly different from that of magnetite crystals formed as rock-forming minerals in inorganic growth environments. For magnetite crystals formed by biological activities, simple octahedral Tracht is seldom observed; they are mostly cubo-octahedral with {100} or cubo-dodecahe- dral with {110}, but rarely take one directionally malformed conical weight-like form. These crystals align to form a single rosary chain. Magnetite is a mineral belonging to the spinel group, and has a reverse spinel structure. The {111} face is the close-packed plane of oxygen, and the structural form should be simple octahedral bounded by {111} only, not only from the BF law and the DH expanded law, but also by HP PBC analysis. Magnetite crystals formed in an inorganic environment as a rock-forming mineral almost exclusively take this form. The {100} and {110} faces are encountered only exceptionally. There is a report that {100} faces occur only when crystals are smaller than 0.5 m.

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